The present invention generally relates to tools used in the surveying and construction industries. More particularly, the present invention relates to tools, such as plumb bobs, used to establish vertically, or laterally, aligned points with respect to a predetermined datum.
In surveying and construction, it is quite often necessary to establish a point, or set of points, which vertically, or laterally, align with a preexisting point. To accurately establish these points, a tool termed a plumb bob is commonly used. A plumb bob is basically comprised of a heavy weight attached to one end of a string. A worker holds the string such that by the force of gravity the weighted member vertically aligns itself with the preexisting datum. The string then becomes a vertical extension of this datum. On the construction site, the topography of the ground and the botanous growth sometimes prohibit workers from clearly seeing certain datums from a distance. Therefore, a surveyor using a transit may construct a set of laterally aligned points by focusing on the string instead of the datum itself. In a second scenario, a worker constructing a door jamb or similar edifice may extend the string to an overhead structure to determine a point which is vertically aligned with the preexisting datum. In any case, the plumb bob provides a highly accurate tool with which a plurality of vertically and/or laterally aligned points may be established.
Traditionally, plumb bobs have been elongated and symmetrically shaped about a central, linear axis. Typically, a frusto-conical geometry (i.e., a cone tapering inwardly from a maximum diameter at its top to a minimum diameter, or point, at its bottom) is employed in the construction of plumb bobs. Due to its vertical symmetry and even distribution of weight, this shape permits the string to become a highly precise extension of the preexisting datum. Several examples of such plumb bobs may be found in U.S. Pat. No. 2,791,037 to Damron, issued May 7, 1957; and U.S. Pat. No. 4,461,091 to Gammon issued Jul. 24, 1984, as well as British Patents No. 614,735 to Suverkrop, published Dec. 22, 1948; and 1,320,677 to Florin, published Jun. 20, 1973.
The only inaccuracy which may be found with this commonly shaped plumb bob is that of an inaccurate alignment of the plumb bob itself with respect to the preexisting datum. Due to its inwardly tapering, solid body, a worker sighting from above is incapable of seeing both the bottom of the plumb bob and the preexisting datum. To help resolve this inaccuracy, a second worker is sometimes utilized. This second worker kneels on the ground beside the preexisting datum and directs the worker holding the string until the bottom of the plumb bob is substantially vertically aligned with the preexisting point.
In addition to the inaccuracy inherent in prior art plumb bobs, another undesirable feature of the tool is the inability to house the entire flexible line in a reel housing when the plumb bob is not in use. U.S. Pat. No. 3,172,205 to Gammon attempts to protect the line by retracting it into a reel housing when not in use, but this reel fails to house the portion of the line immediately adjacent the top surface of the plumb bob. This causes that particular portion of the line to fray and eventually break, thereby necessitating cutting a small portion of the line and retying it to the plumb bob. Eventually, the line becomes too short to use, thereby rendering the reel useless.